This invention relates to improvements in impellers and devices (e.g., fans) incorporating same.
Forced transport of fluid is commonly achieved through the use of a centrifugal or axial flow fan. An axial flow fan impeller consists of a common propeller like component for drawing fluid (typically air) in from one side and out through the other side. The fluid travels substantially in a straight line along the propeller""s axis assisted by the shape and construction of the propeller housing. In contrast the impeller of a centrifugal fan is wheel-like in appearance and the outgoing fluid travels in a direction substantially perpendicular to the axis of rotation.
The efficiency characteristics (directly related to the running cost) of both types of fans are determined by the number, size, shape and general fluid dynamic properties of the blades which comprise the fan. The operating speed and impeller housing can also have a marked effect on the efficiency. Common fans or pumps of both types have been found to be relatively expensive and/or noisy in operation.
The object of the present invention is primarily concerned with improving the efficiency of the impellers used in fans or pumps. Another significant object of the invention is to provide an impeller with substantially less operating noise.
The present invention provides a third class 6f fan and fan impeller. For a somewhat descriptive name, the fan can be called a multiflow fan, i.e., one which combines the properties of axial with transverse flow. The multiflow impeller has some similarities to centrifugal impellers but differences in the shape and orientation of its blades means that it is has a minimal or no centrifugal effect on the air flow through the fan.
The multiflow fan of the present invention can produce a high output at high efficiency. That means the costs for power used in running the fan may be reduced below those of a centrifugal fan having an impeller of the same diameter and having the same output. The power input is to a large degree constant at high volume flows and static pressure is maintained. The multiflow fan can be operated at a reduced speed and there are no unstable operating regions in the performance curve at any speed. It is an advantage that the impeller can be used with a conventional scroll-type housing or any other housing configuration that is suitable for a centrifugal impeller or in tubular or rectangular casing suitable for axial impellers.
In one broad aspect of the invention there is provided a fan impeller having an axis about which the impeller is rotatable in a working direction of rotation, and a plurality of aerofoil blades lying spaced from and arranged about the axis, the inward axis-facing surface of each blade defining a longer fluid flow path across the blade than the opposite outward axis-facing surface of the blade, and with each blade having an angle of attack from 0xc2x0 up to a positive angle of attack less than that at which the blade will induce turbulent fluid flow when the impeller is rotated in a fluid at a working speed in the working direction of rotation, whereby rotation of the impeller in the working direction of rotation induces an inlet fluid flow generally axially towards the impeller and an outlet fluid flow away from the impeller in directions generally inclined about 30xc2x0 or more or less relative to the axis.
In a second broad aspect of the invention there is provided a fan comprising a housing having an inlet and an outlet and a fluid flow path between the inlet and the outlet, an impeller as defined above mounted in the flow path within the housing to be rotatable about its axis, and drive means enabling the impeller to be rotated in its working direction of rotation to cause fluid flow from the inlet to the outlet of the housing.
The aerodynamic properties of the blades of the multiflow impeller described herein are similar to those of an aircraft wing and can be likened particularly to when the aircraft is performing a loop or inward turn.